Pressure sensor and soil stress isolation filter arrangement in a pore pressure probe

Abstract

A system for measuring transient pore water pressure in the ground utilizes a probe member having a unique arrangement of a pressure sensor and a soil stress isolation filter. The probe member has a body portion with a hollow cavity defined therein. The pressure sensor in the form of a ceramic transducer is mounted in the cavity, and the filter in the form of a circumferential gap formed on the body communicates with the cavity. The filter is also configured for isolating the sensor from effective soil fabric stresses while allowing access to the transducer by pore water pressure. A coupling medium in the form of silicon grease fills the cavity so as to transmit the pore water pressure to the transducer.

Having thus described the invention, what is claimed is: 
     
       1. In a system for measuring transient pore water pressure in the ground, a probe member comprising: (a) a body portion having a hollow cavity defined therein and including an inner tip section and an outer tip section, said inner tip section having a lower hollow end, said outer tip section having an upper hollow end and adapted to be connected to said inner tip section at its lower hollow end such that said hollow cavity is formed between said inner and outer tip sections at the connection of said sections to one another;   (b) a pressure sensor mounted in said cavity;   (c) said inner and outer tip sections of said body portion having adjacent generally planar parallel spaced surfaces which define a filter in the form of a narrow circumferential gap disposed on said body portion at said connection of said inner and outer tip sections and open to the exterior of said body portion circumferentially thereabout, said circumferential gap defined between said parallel spaced surfaces of said inner and outer tip sections of said body portion being formed by bottoming-out of a recessed shoulder of said upper hollow end of said outer tip section on a flat annular inner surface of said lower hollow end of said inner tip section, said outer tip section further having a plurality of radially-extending holes defined therein in alignment with said circumferential gap and open to both said gap and said cavity for providing communication of pore water located about the exterior of said body portion with said cavity therein via said circumferential gap and said radially-extending holes while isolating said cavity and said sensor mounted therein from effective soil fabric stresses; and   (d) a coupling medium disposed in said cavity between said sensor and said filter for transmitting pressure produced by said pore water to said sensor.   
     
     
       2. The probe member as recited in claim 1, wherein said plurality of radially-extending holes are defined through said recessed shoulder of said upper hollow end of said outer tip section in alignment with said circumferential gap. 
     
     
       3. The probe member as recited in claim 1, wherein said pressure sensor is a piezoelectric ceramic pressure transducer having a rectangular plate-like configuration. 
     
     
       4. The probe member as recited in claim 3, wherein said transducer is protected by a coating of epoxy resin potting and a coating of parafin wax to provide an impervious moisture barrier. 
     
     
       5. The probe member as recited in claim 1, wherein said coupling medium is silicone grease.